This question seems to be hypothetical. As @rolfl said, legitimate duplicates are extremely rare, and I don't think that's going to change in the foreseeable future.

That being said, duplicates might happen every once in a while, in theory at least, and I agree that the current message is completely useless, and we need something better, just in case.

How about something like this? It's intentionally a bit verbose, feel free to trim to your taste:

This question has been asked before and already has an answer. If those answers do not fully address your question, please explain your reasons, and what more you are looking for. One way or another, you need to differentiate from the previous question.

Keep in mind that the question might be hard to answer. To encourage better reviews, you can try to offer a bounty, or share the question on various social outlets such as Twitter, Facebook, or others.

Yeah, the comment "your question is a duplicate so we closed it, please ask again" doesn't make any sense in its current form. However, let's keep in mind that the situation is quite hypothetical, because as @rolfl said it too, legitimate duplicates are extremely rare on Code Review, and I don't think that's going to change in the foreseeable future.

That being said, duplicates might happen every once in a while, in theory at least, and I agree that we need a better text, just in case.

How about something like this? It's intentionally a bit verbose, feel free to trim to your taste:

This question has been asked before and already has an answer. If those answers do not fully address your question, please explain your reasons, and what more you are looking for. One way or another, you need to differentiate from the previous question.

Keep in mind that the question might be hard to answer. To encourage better reviews, you can try to offer a bounty, or share the question on various social outlets such as Twitter, Facebook, or others.